Holmium is a chemical element with the symbol Ho and atomic number 67. Part of the lanthanide series, holmium is a rare-earth element. Elemental holmium is a relatively soft and malleable silvery-white metal. It is too reactive to be found uncombined in nature, but it is relatively stable in dry air at room temperature when isolated. However, it reacts with water and corrodes readily, and also burns in the air when heated. Holmium has the highest magnetic permeability of any element and is thus used for the polepieces of the strongest static magnets. Because holmium strongly absorbs neutrons, it is also used as a burnable poison in nuclear reactors.
 

Lanthanide Chemical Elements

The lanthanide or lanthanoid series of chemical elements comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium.

These fifteen lanthanide elements [Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb), Lutetium (Lu)], along with the chemically similar elements scandium and yttrium, are often collectively known as the rare earth elements.